testing/fake/gnmi: use Path.Elem instead of deprecated Path.Element

[timehacker]

Problem

valToResp() in the fake gNMI server builds subscription responses with &gpb.Path{Element: val.Path} — the deprecated []string field that was superseded by Path.Elem ([]*PathElem) in gNMI v0.4 (2018).

Modern consumers that call only p.GetElem() receive an empty path for every update. In practice this means every metric value from a fake-server subscription arrives under the key "/" rather than its real OpenConfig path (e.g. /interfaces/interface/state/counters/in-octets).

Fix

Add stringsToPathElems() which converts the []string path representation from the fake proto config into []*PathElem, correctly extracting key–value pairs from bracketed decorators:

"interface[name=swp1]" → PathElem{Name:"interface", Key:{"name":"swp1"}}

Both the Delete and Update branches of valToResp() are updated to use Path.Elem via this helper.

Also included

• TestStringsToPathElems: unit tests for the new helper covering simple elements, single-key elements, multi-key elements, and empty-string filtering.
• TestValToRespPathEncoding: asserts that responses from valToResp() populate Path.Elem and leave the deprecated Path.Element empty.
• cumulus-linux.pb.txt: a Cumulus Linux 5.x simulation config for the fake_server command (interfaces swp1–swp4, temperature sensors, fans, PSU, QoS buffers) that serves as a realistic test fixture and demonstrates the fix with real-world OpenConfig paths.

Tested

Verified end-to-end: gnmic v0.44.1 subscribing to the patched fake server now sees full OpenConfig paths in subscription responses, enabling downstream consumers to produce correctly named metrics.

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